1. Field of the Invention
This invention relates to the field of storing and retrieving magnetically recorded, appropriately reencoded, physical representations of digitally encoded, logical information on and from moving storage media such as magnetic disks and tapes. More particularly, the invention provides an improved arrangement for the incorporation of the required control and data information into such magnetic representations and improved method techniques and apparatus for implementing the practical application thereof to both disk and tape type media in respect of both writing to and reading from the latter. Still more specifically, the invention provides a uniform and more efficient format for the recording and sensing of magnetic representations of control and data information while using either disk or tape type media, together with method techniques and interfacing, controller apparatus that are employable in identical manner and form for either disk or tape type media. Other advantages of the invention are further described in the appropriate subsequent sections hereof.
2. Description of the Prior Art
The relationship of this invention to the state of the known prior art can be understood in proper perspective only in the context of recognition of the three facets of the invention that cooperate to achieve optimized realization of the advantages it makes possible--namely, the improved arrangement or "formatting" of the physical representation that is magnetically recorded upon a storage medium to provide an efficient and reliably retrievable record embodying both given data and needed control information, the improved combination of information handling and magnetic storage and retrieval steps to provide a method for practical application of the improved arrangement or "formatting", and the improved combination of structural elements to provide practical apparatus for implementing the improved method.
In general, the improved arrangement for the physical representation of information magnetically recorded upon a disk medium is believed to be basically novel and to depart from prior practices in a manner diametrically contrary to long established and universally employed, present formatting conventions. The combinational aspect of the improved method is believed to also be both new and a previously uncharted excursion from accepted prior information storage and retrieval methodology, although the improved method does employ in combination a number of individual steps or techniques which of themselves are each either old or well within the skill of the art when considered in isolation (as to which per se, and apart from the combinational aspect of the improved method provided by this invention, no claim is herein made). Similarly, the combinational aspect of the improved apparatus is believed to represent a significant and novel departure from both prior constructions and prevailing conceptual approaches to implementation of hardware for the interfacing of a computer with moving media type, peripheral, magnetic storage units, although the improved apparatus does employ in combination a number of individual elements or modules which of themselves are each either old or well within the skill of the art when considered in isolation (as to which per se, and apart from the combinational aspect of the improved apparatus provided by this invention, no claim is herein made).
To further illustrate the relationship of the invention to the background state of the art, and with reference first to the apparatus aspect of the matter, it is clear that the computer and the magnetic disk (and optionally also magnetic tape) units, including their internal drive assemblies, magnetic, read/write heads, etc., involved in the overall, systemic combination to which the invention is applicable may not only each be of conventional character, but may be of various specific, known constructions, which is, of course, a desirable attribute of the invention permitting its immediate application in existing data processing systems merely by incorporation therein of the improved form of peripheral magnetic storage controller assemblies contemplated by the invention. Likewise, conventional types of magnetic disk and tape media can be employed in implementing the invention on an immediate basis. Since another advantage of the invention is that it requires no provision for "hard indexing" of disks, however, it will be apparent that the hard indexing means included in conventional disk media and the hard index detecting means included in conventional disk drive units are not utilized in the practice of this invention and may be omitted from disk media and drive units produced in the future, with consequent reduction of production costs and enhanced reliability through simplification. With respect to the improved controller assemblies provided by the invention, the individual, electronic, functional modules employed therein (i.e., a buffer, a write data selector, a 4-bit code to 5-bit code encoder, a complementer or multi-bit inverter, a parallel to serial converter, a write clock, a write counter, a serial to parallel converter, another complementer, a 5bit code to 4-bit code decoder, a read data latch, a synchronizing signal controlled read clock, a read counter, an EXCLUSIVE OR data start signal detector, a 1-bit latches, etc.) are each either conventional from the standpoint of the internal components and circuitry involved in the individual functional module or of nature to be readily constructed according to the desired operating parameters by those skilled in the art with conventional components and, in most instances, in a variety of alternative fashions available as a matter of choice, merely from the functional descriptions of such modules hereinafter contained (if not, indeed, merely from their descriptive names and functional relationships to the other modules). Such amenability of the invention to implementation of the improved apparatus it contemplates by appropriately combining conventional types of individual, electronic, functional modules is thought to be another of its advantages.
Similarly, with reference to the method aspects of the invention, from the standpoint of the individual steps involved, the improved method advantageously largely employs known and well understood techniques. For example, the operations of writing a physical representation of electrical signal levels corresponding to the binary value of "bits" of encoded information onto a moving, magnetic storage medium by correspondingly setting the magnetic state of successive small zones along an elongate recording track of the medium through appropriately controlling the momentary energization of a gapped, electromagnetic "write head" adjacent the track and of subsequently sensing the magnetic states of a succession of such zones to produce an electrical output signal of level varying in response to transitions between the states of such successive zones moving into juxtaposition with a gapped, electromagnetic "read head", are quite conventional. Likewise, various techniques for expressing information in digitally encoded form (such as in ASCII or ISO interchange code or another binary type code internally useable within a computer), for further encoding such computer compatible, encoded information into a special recording code for use in recording the same upon a magnetic storage medium (such as in the recording codes known as "FM", "MFM", "GCR", etc.), for decoding transitions of magnetic state between successive zones of a track of a recording medium serially sensed during reading from the medium into electrical signals encoded in the recording code being employed, and for then further converting from electrical signals encoded in the recording code to electrical signals encoded in the internally recognizable computer code, are all individually well known and widely practiced. Other individually known techniques utilized in certain steps of the improved method include temporarily storing encoded information representations in a buffer memory, controlling the storage of such information representations into the buffer memory or the accessing thereof from the buffer memory in increments of groups of electrical signals representing a preselected number of bits of encoded information in "parallel", converting information representations encoded in one such parallel code into another parallel code (such as from internal code or segments thereof into recording code, or visa versa), converting encoded information representations from a parallel encoding involving a plurality of electrical signals into a series of successive electrical signals whose levels have binary significance and visa versa, timing or "clocking" the recording of information represented as magnetic flux states of successive zones along a track of a magnetic storage medium, deriving an electrical signal pulsed at regular intervals from control information "embedded" in the magnetically recorded version of data information encoded in a "self-clocking" type recording code for use as a synchronizing control signal during reading from a magnetic storage medium, counting the number of binary bit value representations or predetermined groups thereof being written onto or read from a magnetic storage medium, etc.
As previously indicated, therefore, it should be understood that this invention is primarily concerned with overcoming certain very significant limitations and disadvantages of prior approaches to the interfacing of a computer with peripheral magnetic storage units, especially those of the disk type, and that the invention provides a solution to the problems inherent in prevailing interfacing practices through first adopting a radically different arrangement or format for the ultimate, overall, physical representation of information to be transferred from a computer onto the recording medium of a peripheral disk (or tape) storage unit or visa versa, and then conveniently provides for the implementation of such new arrangement or format through the employment of an overall method and overall interfacing apparatus which can be both easily understood and readily placed into practice by those skilled in the art, by virtue of such method and apparatus being predicated upon merely combining in a new way individual method steps and individual electrical modules that are of themselves each familiar and immediately available to those skilled in the art.
At least some of the primary limitations and disadvantages of prior and current practices with respect to the interfacing with computers of peripheral magnetic disk storage units may be summarily noted as further background for appreciation of both the accomplishments of this invention and the marked extent to which the various facets thereof depart from what has heretofore become universally accepted, conventional practice.
Prior disk type, magnetic storage media have employed what is referred to as "hard indexing", typically implemented through the provision of one or more holes, protuberances or the like on the physical rotating medium itself, which can be optically or otherwise sensed when passing by a fixed point within the associated drive unit during rotation of the disk. Another approach to hard indexing has been to magnetically record upon a special track of a disk being initialized for use one or more magnetically represented index "marks" that could be magnetically sensed during rotation of the disk. With either of such hard indexing implementations, however, it has been customary in fulfilling the very purpose of such hard indexes to commence magnetically recording information upon the disk only at a fixed point of the track to be recorded corresponding to the detection of the hard index of the disk or at a subsequent point thereon separated from the hard index by some predetermined time interval of rotation of the disk and a corresponding stretch of the recording track frequently referred to as a "gap" (in which some pattern of flux transitions having control signficance at least for identifying the stretch as a particular gap, but none of the actual data information to be stored, will typically be recorded). Thus, every information writing operation upon conventional disk media involves a statistically inherent average latency interval before the recording of information can commence, since the rotation of the disk into a position bringing the hard index to the point for detection thereof must be awaited.
Prior and current practice has also been to record information upon disk type media, after an initial, common, "post index" gap, in a significant number of so-called "sectors" along each recording track, which are separated from each other by additional gaps contained in each sector and also not containing a physical representation of any of the data information to be stored and retrieved, the latter being limited to representation within the part reserved for data of one or the other of the separate sectors contained in the track. This now "inbred" practice of conventional disk storage methodology and apparatus has apparently survived as a historic remnant of the very early days of magnetic disk storage technology in which it was common for some part of a recording track upon a disk medium to be defective, so that "sectoring" was adopted to facilitate the recording of information upon those parts of a particular disk track that were not defective, while skipping those that were defective. Such long standing practice of sectoring of information recorded upon the track of a disk medium has, however, not only for some time been unnecessary in view of the improved quality of disk media resulting from technological advances in the manufacture of such media, but represents a significant waste of the usable length of recording space available in each track of a disk for the storage of useful data information, as well as tending to complicate problems of providing satisfactory synchronization during reading from the disk. Certain typical, sectored arrangements for the physical representation of information on the track of a disk type storage medium will hereinafter be considered in greater detail in comparison with the improved arrangement provided by this invention, but it may be noted that the difference of storage efficiency is significant, with a typical sectored arrangement utilizing less than 65% of the length of each track for the storage of actual data information, as compared with the improved arrangement provided by this invention in which useful data information may occupy in excess of 95% of the length of each track.
A side effect of the prior adherence to a sectored format for information recorded upon disk type media, which is apparently based upon implied recognition of the inefficiency of the latter, is that an entirely different formatting arrangement has been employed for storage upon tape type media, wherein the constraints of track length inherent in disk type media are not applicable. The current de facto acceptance of the assumed necessity for arranging or formatting the physical representation of magnetically recorded information differently for disk and tape types of media has heretofore also brought about an assumed necessity for employing separate and differing methods and controller assemblies for the storage and retrieval of information upon disk and tape types of media respectively.